For the large-area heating of objects, it is common to pass the objects through a heating tunnel, furnace, kiln or lehr which can have heaters disposed along the path of the objects to be heated and spacedly juxtaposed with them for radiant heating of the objects. This technique has been used for a wide variety of purposes, including the annealing or tempering of glass, the heating of glass to impart curvature thereto, for the curing of surface coatings, for the drying of coatings, for the drying of objects and for a variety of thermal treatments of ceramic, glass, metal or composite articles.
Reference may be made, for example, to U.S. Pat. Nos. 4,531,047 and 4,601,743 by way of example of such heating paths.
In those systems, quartz heaters are used for careful control of the heating and the object utilizing electricity as the energy source. Electrically energized radiant heaters provide an opportunity for fine control of the heating operation.
It is also known to utilize other energy sources for heating in similar applications. For example, a combustible fuel may be supplied to a burner to generate a flame which is employed to heat the object.
Where radiant heating is desired, a burner of the type described in U.S. Pat. No. 4,547,148 may be used. That patent describes a radiant gas-fired burner having a back plate for connection for a supply of air and gas which, together with a combined refractory fiber pad and metal support structure, defines a plenum to which the combustible air/gas mixture is supplied. A perforated metal member is interlocked with the porous pad of refractory fibers as a support for it and through the perforations of this member, the fuel/air mixture is supplied to the porous layer so that combustion can be sustained at the surface thereof which can be spacedly juxtaposed with an object.
Both gas burners and electrically-energized radiant heaters have advantages and disadvantages for most of the applications described. For example, a gas burner can deliver radiant energy at two to three times the delivery rate of electrical heaters but has the drawback that the heating cannot be controlled or regulated with great precision. Electrical heaters have the advantage that they can be controlled with precision and both types of heaters frequently are characterized by a slow cool down rate.
Electrical heaters generally have start-up inertia, i.e. cannot be brought to the optimum temperature level rapidly. As a consequence, it has been necessary for the design of a heating tunnel or furnace to be a compromise based upon these different characteristics.